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Intro to CLR ThreadPool Growth

ThreadPool Growth: Some Important Details

The CLR ThreadPool has two types of threads - "Worker" and "I/O Completion Port" (aka IOCP) threads.

  • Worker threads are used when for things like processing Task.Run(…) or ThreadPool.QueueUserWorkItem(…) methods. These threads are also used by various components in the CLR when work needs to happen on a background thread.
  • IOCP threads are used when asynchronous IO happens (e.g. reading from the network).

The thread pool provides new worker threads or I/O completion threads on demand (without any throttling) until it reaches the "Minimum" setting for each type of thread. By default, the minimum number of threads is set to the number of processors on a system.

Once the number of existing (busy) threads hits the "minimum" number of threads, the ThreadPool will throttle the rate at which is injects new threads to one thread per 500 milliseconds. This means that if your system gets a burst of work needing an IOCP thread, it will process that work very quickly. However, if the burst of work is more than the configured "Minimum" setting, there will be some delay in processing some of the work as the ThreadPool waits for one of two things to happen 1. An existing thread becomes free to process the work 2. No existing thread becomes free for 500ms, so a new thread is created.

Basically, it means that when the number of Busy threads is greater than Min threads, you are likely paying a 500ms delay before network traffic is processed by the application. Also, it is important to note that when an existing thread stays idle for longer than 15 seconds (based on what I remember), it will be cleaned up and this cycle of growth and shrinkage can repeat.

If we look at an example error message from StackExchange.Redis (build 1.0.450 or later), you will see that it now prints ThreadPool statistics (see IOCP and WORKER details below).

System.TimeoutException: Timeout performing GET MyKey, inst: 2, mgr: Inactive, 
queue: 6, qu: 0, qs: 6, qc: 0, wr: 0, wq: 0, in: 0, ar: 0, 
IOCP: (Busy=6,Free=994,Min=4,Max=1000), 
WORKER: (Busy=3,Free=997,Min=4,Max=1000)

In the above example, you can see that for IOCP thread there are 6 busy threads and the system is configured to allow 4 minimum threads. In this case, the client would have likely seen two 500 ms delays because 6 > 4. Note that these ThreadPool usage stats are for the entire client app and are not only threads used by StackExchange.Redis.

Note that StackExchange.Redis can hit timeouts if growth of either IOCP or WORKER threads gets throttled.


Given the above information, we strongly recommend that customers set the minimum configuration value for IOCP and WORKER threads to something larger than the default value. We can't give one-size-fits-all guidance on what this value should be because the right value for one application will be too high/low for another application. This setting can also impact the performance of other parts of complicated applications, so each customer needs to fine-tune this setting to their specific needs. A good starting place is 100, then test and tweak as needed.

How to configure this setting:

  • In ASP.NET, use the "minIoThreads" configuration setting under the <processModel> configuration element in machine.config. If you are running inside of Azure WebSites, this setting is not exposed through the configuration options. You should still be able to set this programmatically (see below) from your Application_Start method in global.asax.cs.

Important Note: The value specified in this configuration element IS a per-core setting. For example, if you have a 4 core machine and want your minIOThreads setting to be 200 at runtime, you would use <processModel minIoThreads="50"/>.

Important Note: The value specified in when calling SetMinThreads IS NOT a per-core setting. If you want the setting to be 200 at runtime, pass 200 to this API regardless of the number of cores the machine has.


Hey Jon,

Wondering if you could help me think about this correctly.

I thought I understood how adjusting the min threads would help with timeouts on thread pool growth. After reviewing our error logs I found the highest Busy values for IOCP and WORKER pools and set the minimums to be slightly higher values. About an hour after deploying, we received this exception...

System.TimeoutException: Timeout performing EVAL, inst: 1, mgr: Inactive, err: never, queue: 3, qu: 0, qs: 3, qc: 0, wr: 0, wq: 0, in: 106, ar: 0, IOCP: (Busy=60,Free=940,Min=100,Max=1000), WORKER: (Busy=192,Free=32575,Min=500,Max=32767)

I would have thought this would be an impossible error to have less busy than the minimum. Do you know if it doesn't actually create the thread pool with the minimum threads and just treats the minimum as what to not let it drop under once it scales up to that level?


JonCole commented Feb 22, 2016

Busy can be less than Min. The system doesn't keep Min threads around at all time - it just is willing to quickly scale up to Min without any delays. After it hits Min, it will start throttling the growth.

In your case, you hit a timeout but it is not likely to be because of thread pool growth throttling - something else is causing the timeout. For instance, client side CPU usage spikes to 100% (e.g. threads have delays between access to the CPU). See and for more details.


Awesome post! This solved our problem with massive numbers of timeouts with Azure Redis Cache using the StackExchange.Redis library.

marsen commented Nov 17, 2016

System.TimeoutException: Timeout performing GET MyKey, inst: 2, mgr: Inactive,
queue: 6, qu: 0, qs: 6, qc: 0, wr: 0, wq: 0, in: 0, ar: 0,
IOCP: (Busy=6,Free=994,Min=4,Max=1000),
WORKER: (Busy=3,Free=997,Min=4,Max=1000)

HI I want to know those parameters mean ,
inst mgr queue qu qs qc wr wr in ar ,
Is any article or document can share to me ?

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